164 OUTLINES or EQUINE ANATOMY. 



double fold of membrane, wliicL serves to bring about a com- 

 munication between the cavities of the cochlea and of the 

 tympanum. On the bony plate which separates the scalce 

 (lamina sjnralis) the fibres of the auditory nerve are spread 

 out ; they terminate in cells which are situated in the scala 

 media (fibres or rods of Corti), " arranged like the key- 

 board of a piano." On reaching the internal ear, after pass- 

 ing through foramen auditorium internum, the auditory, 

 portio mollis, or eighth pair of nerves breaks up into three 

 parts. One runs to the ampullae, another to the membranous 

 vestibule, the third passing into the modiolus gives ofP 

 fibres outward into the lamina spiralis. The intensity of 

 a sound is supposed to be distinguished in the vestibule, 

 its " tone " in the scala media of the cochlea. The ves- 

 tibule is connected with the cavity of the tympanum by 

 the fenestra ovalis, which in the fresh subject is closed by 

 mucous membrane and by the base of the stapes. It will 

 be observed that the cochlea only of the bony labyrinth has 

 its membranous counterpart attached to the bony walls, the 

 membranous semicircular canals and vestibule being sepa- 

 rated from the bone by perilymph. 



The middle ear or tympanic cavity is situated between 

 the external and internal ears. Its outer wall presents the 

 membrana tymimm, which closes the foramen auditorium 

 externum. By the foramen styloideum it is connected 

 ■with the pharynx through the medium of the Eustachian 

 tube. It communicates inferiorly wdth certain cavities in 

 the petrous temporal bone (similar to the sinuses of the 

 face) the mastoid cells. Through this cavity the seventh 

 nerve passes ; it enters through foramen auditorium in- 

 ternum, and during its passage gives off the chorda tym- 

 pani, which runs to the gustatory branch of the inferior 

 maxillary division of the fifth, and also branches to the 

 muscles of the middle ear ; then it emerges by passing 

 through foramen styloideum. The inner wall of the middle 

 ear presents two foramina — fenestra rotunda, leading to the 

 scala tympani ; fenestra ovalis, leading to the vestibule. 

 Into the latter fits the base of the stapes, one of a chain of 

 bones which extends across the tympanic cavity to the 

 membrana tympani. This bone is " stirrup shaped," and 

 its arch superiorly is connected by fibro- cartilage with a 

 portion of the incus. In this joint an osseous nodule may 

 be observed, os orbiculare, commonly enumerated as " th& 



